What is "Force PASV IP"

In the settings>connection I see the "Force PASV IP"
What is that exactely?

I was unable to connect to a Friend FTP server
(Serv-U FTP-Server v2.5d)
He also run Linksys Router with FTP port
27910
27909
Redirected to server LAN computer.

I also run a Hardware Router.....
When I turned that "Force PASV IP" on I could connect!
Else I got no dir listing.....

WHY!!!

Try to disable -La on dir listing options at transfer on settings.

tried that, and it's the same problem!!!
Cant get Dir listing...

[14:42:16] LIST -a
[14:42:19] 150 Opening ASCII mode data connection for /bin/ls.
[14:42:49] Connection closed. Server timeout.
[14:42:49] 426 Data connection closed, transfer aborted.

will ask him to set up a Passive mode Port Range
in his Router!..and server ..Dito!!